Many areas of nanotechnology utilize focused charged particle beams as precision tools, for altering a work piece by scanning the charged particle beam over a work piece in a pattern to remove material, with or without an etch precursor gas, or to add material through deposition of the charged particle beam or by beam-induced decomposition of a deposition precursor gas. The progress of the milling or deposition process during the formation of patterns is often monitored by an imaging process. Typically, during milling and deposition, the focused charged particle beam is vectored across the surface of the substrate in a predetermined pattern. This vectoring process is accomplished by beam deflectors which generate electrostatic or magnetic dipole fields directed perpendicularly to the direction of beam travel. Usually, means are also provided for turning the beam on and off, a process called “beam blanking”. The milling process utilizes the high atomic weight and high energy of the charged particles in the beam to sputter away atoms from the substrate, thereby “milling” a pattern into the substrate surface, wherein the milled pattern corresponds to the trajectory of the charged particle beam across the substrate surface. The deposition process may utilize lower energy beams of atomic or molecular ions comprising a material to be deposited on the substrate surface, wherein the deposited pattern corresponds to the trajectory of the ion beam across the substrate surface. In other embodiments, the deposition process may utilize electron beams which induce the decomposition on the surface of a deposition precursor gas. In many cases, it is not possible to precisely calculate the rate of material removal or deposition due to the charged particle beam, thus some means of monitoring the etching or deposition progress is desirable. This monitoring process is called “imaging”, wherein the surface is typically scanned with a beam in an X-Y raster or serpentine pattern to generate some form of imaging signal which is then used to modulate the intensity of a display screen to provide an image of the surface where the milling or deposition process has occurred. There are a number of methods in the prior art which have been employed for the imaging process, as described below.
One method for combining milling and imaging processes within a single system is a focused ion beam (FIB) system employing an alloy ion source and an in-column mass spectrometer. An example of such a system is described in U.S. Pat. No. 4,929,839, issued May 29, 1990, “Focused Ion Beam Column”. A liquid metal ion source is employed, wherein the liquid metal is an alloy containing two or more atomic species. One example is a silicon-gold eutectic alloy, wherein the eutectic melting temperature is very low compared with the melting temperatures of either silicon or gold (a “deep” eutectic). The beam produced by the ion source contains both silicon and gold ionic species, often with both one and two positive charges per ion. In this FIB column, an E×B (“Wien filter”) is located between the first and second electrostatic lenses. By proper adjustment of the crossed electric and magnetic fields in the E×B filter, it is possible to select a single ion species from the ion beam entering the filter to pass through the E×B filter undeflected, wherein all other ion species are deflected off-axis, striking an aperture, and thus being eliminated from the beam that is focused onto the substrate surface. For milling, either single- or doubly-ionized gold ions can be selected for the beam at the substrate. The large atomic weight of the gold ions generally produces very high milling rates. For imaging, different electric and/or magnetic field strengths are set in the E×B filter, causing singly- or doubly-ionized silicon ions to be selected for the beam at the substrate. The low atomic weight of the silicon ions produces relatively low sputtering rates, thereby enabling imaging of the substrate with little damage. This approach to combining milling and imaging within a single tool has the advantage of rapid switching between milling and imaging since changes to the E×B filter electric and/or magnetic field strengths may be made rapidly. Disadvantages include the need for expensive and short-lived alloy liquid metal sources, and the cost and added complexity of the E×B filter in the column.
Another approach for incorporating both milling and imaging in a single tool is exemplified by the FEI Company Expida 1255S dual-beam system. In this tool, two separate columns are employed: a high-resolution electron beam column for imaging, and an ion beam column for milling. An obvious advantage of this approach is that since separate columns are used for the two functions, it is possible to separately optimize the columns for their individual roles: the ion column is used only for milling (not imaging), while the electron column is only for imaging (not milling). A disadvantage of this approach is the added cost and complexity of two columns, and the need to precisely move the substrate from under one column to under the other column Any relative positioning errors between the two columns may result in positioning errors of the milling beam relative to the desired location of the pattern to be milled.
Another approach to combining milling and imaging in a single system is to simply use a single column, with a source containing a liquid metal such as Gallium. In this case, the source and column operating parameters for the two modes may be identical. The difference between the milling mode and the imaging mode is solely determined by the way the beam is moved across the substrate surface. For example, during milling, the beam might be vectored across the substrate surface to a number of locations in sequence, and allowed to dwell at each location for a time sufficient to induce appreciable material removal by sputtering from the substrate. During imaging, the beam would be rastered across the substrate surface with dwell times at each pixel much shorter than for the milling operation, thereby minimizing substrate milling during imaging. For this approach, the liquid metal ion source (LMIS) runs at a fixed source performance (angular intensity and brightness) by maintaining a constant operating temperature and a fixed extraction voltage and heater filament current. In some cases, different beam-defining apertures may be employed for the milling and imaging modes, enabling high spatial resolution for imaging and higher substrate sputtering rates (at lower spatial resolutions) during milling. However, since for the imaging mode so much of the beam current strikes the beam-defining aperture, undesirable aperture erosion may occur. An advantage of this approach is the simplicity of the ion source and column. A clear disadvantage is the fact that imaging may also produce appreciable substrate milling (over the area of the raster pattern), thereby inducing undesirable damage to areas not intended to be milled—such damage may be unacceptable for many applications such as milling of semiconductor devices and MEMS structures.